The invention relates to a device for ventilating a fuel tank of an internal combustion engine, in particular of a fuel tank of an internal combustion engine of a motor vehicle.
The fuel tank of motor vehicles is normally equipped with a tank ventilation device. On one hand, the tank ventilation device prevents that, in the case of a significant rise or drop of ambient temperatures, an undesired overpressure or negative pressure forms in the pressure-tight closed tank as a result of the evaporation of fuel caused by the rise of temperature or, respectively, condensation of fuel vapors caused by the drop of temperature. On the other hand, the tank ventilation device allows a gas mixture which is displaced by the fuel during refueling, to escape from the interior of the fuel tank. To prevent an undesired leaking of hydrocarbons into the atmosphere or the environment in the mentioned cases, modern tank ventilation devices include an activated carbon filter which is usually connected to the fuel tank by a first gas line, to the atmosphere or environment by a second gas line and to an intake tract of the internal combustion engine by a third gas line. The first gas line serves for ventilating the fuel tank and contains at least one tank shut-off valve, which automatically opens at a predetermined overpressure or negative pressure in the fuel tank and is opened by the control unit during refueling of the fuel tank to conduct the displaced gas mixture through the activated carbon filter. The gas mixture which during refueling and/or when the tank shut-off valve is opened, flows out of the fuel tank into the activated carbon filter where it is cleared of hydrocarbons, is conducted out into the atmosphere through the second gas line.
The second gas line contains a ventilation and aeration valve, which is opened by the control unit when the fuel tank is ventilated, and also when the tank shut-off valve is opened and as a result of negative pressure in the fuel tank, air is aspirated from the environment through the second gas line and the activated carbon filter into the fuel tank or, when air is aspirated from the environment through the second gas line, the activated carbon filter and the third gas line into the intake tract of the internal combustion engine when regenerating the activated carbon filter during operation of the internal combustion engine, to flush the activated carbon filter clear of the adsorbed hydrocarbons and to conduct the hydrocarbons together with the aspirated air into the combustion chambers of the internal combustion engine for burning. The third gas line contains a regeneration valve which is opened by the control unit when the activated carbon filter is regenerated.
During normal operation of a motor vehicle with an internal combustion engine, the activated carbon filter is regenerated regularly, because the internal combustion engine is frequently operated under load conditions which allow a regeneration of the activated carbon filter. In contrast, the load conditions appropriate for regenerating the activated carbon filter occur less frequently in motor vehicles with hybrid drive or in so-called plug-in motor vehicles with an electric motor which serves as drive engine and an internal combustion engine which is only carried along as auxiliary motor and for recharging the vehicle battery. This can have the consequence that an activated carbon filter which is loaded with a larger amount of hydrocarbons cannot be regenerated over an extended period of time. Similar considerations apply to motor vehicles with automatic start-stop mechanism, because in this case the idle state which is appropriate for regeneration of the activated carbon filter occurs less frequently. In such a case, an undesired release of hydrocarbons from the activated carbon filter through the second gas line into the atmosphere or the environment may occur. To avoid this, the activated carbon filter could be dimensioned greater which, however, is disadvantageous with regard to costs and installation space.